Mutagen-sensitive mutants defective in DNA repair mechanisms are collected in Drosophila melanogaster. The mutants are characterized cytogenetically in order to gain a basic understanding of the genetic control of sensitivity to mutagenic agents. The tests used in the initial characterization of these mutants include genetic and cytogenetic mapping, complementation analysis, tests for sensitivity to unrelated mutagens, and tests for pleiotropic effects on related functions such as recombination. A fine structure map of the mei-41 region has been constructed to ascertain the allelism relationship between mus104 and mei-41, and to confirm the large size of mei-41 found during mutational analysis. Many mei-41 alleles are temperature sensitive. A sample of temperature sensitive alleles already mapped is seen to be distributed throughout the locus.